This invention relates to an inverting negative voltage DC-DC power supply circuit and, in particular, relates to an inverting negative voltage DC-DC power supply circuit that achieves a reduction in the number of external components or parts.
Generally, in an inverting negative voltage DC-DC power supply circuit, a reference potential of a resistor serving to set an output voltage should be set to a positive reference potential, and not to a ground potential (zero volts), in order to prevent a negative voltage from being applied to a control IC.
Referring to FIG. 1, a specific example of a conventional inverting negative voltage DC-DC power supply circuit will be described. This example is disclosed, for example, in non-patent document 1 (“HA16114 Datasheet (RJJ03F050-0300Z)”, published by Renesas Technology, Rev. 2.0, Sep. 18, 2003, 37-2 in page 9, FIG. 2.2 Output Voltage Setting (2) in Negative Output Voltage (Vo<0)).
A control IC 100 is connected to an input line from an input terminal 18. The power supply circuit is configured such that a change in voltage at a negative-voltage output terminal 17 is applied to a feedback terminal 105 of the control IC 100 as a positive voltage by means of a voltage setting resistor (Rc) 101, a voltage setting resistor (Rd) 102, and a reference voltage source 10. For this purpose, one end of the voltage setting resistor (Rd) 102 is connected to the reference voltage source 10 through a terminal 106.
Further, a second reference voltage generated by the reference voltage source 10, a resistor (Re) 103, and a resistor (Rf) 104 is applied to a terminal 107. For this purpose, one end of the resistor (Re) 103 is connected to the reference voltage source 10 through the terminal 106. The voltage at the terminal 105 and the voltage at the terminal 107 are compared by an error amplifier 11. The result of the comparison by the error amplifier 11 is input into a DC-DC control circuit 12. The DC-DC control circuit 12 performs control so that the voltage at the output terminal 17 becomes constant. That is, the DC-DC control circuit 12 is a circuit that performs control to make the output voltage constant. Numeral 108 denotes a ground terminal.
The foregoing power supply circuit has the following problem. In order to feed back a change in output negative voltage to the DC-DC control circuit 12, the reference potential of the voltage setting resistor (Rc) 101 and the voltage setting resistor (Rd) 102 should be set to a positive potential (Vref) provided by the reference voltage source 10, and not to zero volts. Because of this, the resistor (Re) 103 and the resistor (Rf) 104 are additionally required for generating a voltage Vref2, satisfying a relationship of Vref>Vref2, as a reference voltage for the control. The reason is to prevent a negative voltage from being applied to the terminal of the control IC 100.